The invention relates to a method and apparatus for covering a roll core that can be metallic or non-metallic with a resin infused fiber reinforcement mat under layer and a polymeric top layer, the top layer preferably formed of a high performance thermoplastic material. However, the present invention is not limited to the use of high performance thermoplastic materials and contemplates the use of other extrudable elastomers such as rubbers or processable urethanes and high viscosity thermoset resins such as polyurethanes and epoxies in general. The resin used for infusion is epoxy or other preferable resins such as cyanate ester, vinyl ester, phenolic, and other low viscosity thermoset systems. The reinforcment fiber mats are preferably made of glass fibers, carbon fibers, aramid fibers (e.g., Kevlar fibers), or other mineral/metallic high strength fibers. These covered rolls are utilized in many applications including an application known as calendering. Calendering is the act of pressing a material, e.g., cloth, rubber, plastics or paper, between rollers or plates in order to smooth or glaze or to thin into sheets. The covered rolls that are discussed in this application are also known as calender rolls, soft-nip calender rolls and supercalender rolls and are often utilized in industrial environments such as paper mills. However, the invention described herein is not limited to covered rolls known by these names or utilized in these environments.
In a typical paper mill, large numbers of rolls are used not only for transporting the web sheet which becomes paper, but also for calendering or processing the web sheet into a specific grade of paper. The finished paper product must possess certain quality characteristics such as a high degree of caliper uniformity, bulk uniformity, smoothness, gloss and printability. In order to achieve these quality characteristics, it is necessary that the calender roll is precisely manufactured utilizing materials that can withstand severe mechanical and chemical conditions during paper processing.
For example, when used for transporting the web sheet during paper processing, it is essential that these covered rolls provide traction to enable the transport of the paper. Additionally, these rolls must be wear and corrosion resistant. During use for calendering, these covered rolls are subjected to high dynamic stress, heat, speed, abrasion and impact and therefore must be fabricated to withstand these elements. In order to function properly for these specific uses, the covered rolls must have an appropriate surface hardness based upon the intended application for the covered roll and also have a high thermal resistance to withstand high temperatures and pressures in the environments in which they are employed. Regardless of their application, these covered rolls are precision elements of the systems in which they are utilized and therefore must be precisely manufactured to achieve balance, specific size and shape specifications, surface characteristics and tight tolerances. The covered rolls have similar transporting and calendering functions in the textile industry as well as in facilities where magnetic tape is manufactured.
Conventional prior art calender rolls comprise a metal cylinder to which either a cotton-filler or a thermoset composite layer (or layers) is added to preclude metal-to-web-to-metal contact at the nip between the calender rolls during the calendering operation. Though cotton-filled roll covers have been used for a long time there are several drawbacks associated with their use such as the need for frequent regrinding. Moreover, cotton filler material is not sufficiently tough to withstand the high stress, high impact requirements and high temperatures that are associated with demanding applications such as in modern paper fabrication. Paper mills must frequently regrind and replace cotton-filled roll covers, even when they perform well. This results in significant production down-time and high costs associated with keeping replacement rolls in inventory.
Over the last two or three decades, synthetic composite roll covers have been developed to resolve many of the problems associated with cotton-filled roll covers. Most of these synthetic composite roll covers use some form of thermoset resin such as epoxy, rubber or polyurethane among others, as a base material which is combined with some form of reinforcement material to improve strength.
As an example, a synthetic composite roll cover is formed of a single layer of reinforcement fiber mat that is impregnated with a thermoset epoxy which is then cured. The surface of the cured single layer is then machined to a smooth finish in accordance with customer specifications.
Alternatively, rather than machining the cured single layer to a smooth finish, a different additional layer of reinforcement fiber mat may be added over the cured single layer, the additional layer being impregnated with an epoxy which is then cured to form a top layer. The surface of the cured top layer, which provides the outer surface of the roll with better characteristics is then machined to a smooth finish in accordance with customer specifications. The single layer which forms an under layer, provides a transitional element between the metal core and the top layer to assist in establishing an effective bond and stress distribution between the two layers of the covered roll. Alternatively, additional layers could be added.
In practice, a layer of the synthetic composite cover is added to the roll core by unspooling a strip of dry reinforcement fiber mat, several inches in width, from a reel and conveying the unspooled strip through an epoxy bath. The roll core is oriented horizontally and rotated to wind the epoxy impregnated strip onto the roll core helically in a back and forth fashion over the roll core length several times until a desired thickness is reached. The epoxy is then allowed to cure to form a layer which is then machined to a smooth finish.
The use of these synthetic composite roll covers has increased dramatically in the last ten years because of their superior performance characteristics over conventional cotton-filled roll covers. The acceptance and usage of synthetic composite roll covers in the paper industry has resulted in the beginning of the demise of the cotton-filled roll cover. Notwithstanding their superiority over cotton-filled roll covers, synthetic composites such as thermosetting epoxies also suffer from several drawbacks. For example, to formulate a synthetic composite roll cover having certain desirable properties such as high toughness, high temperature capability, i.e., high glass transition temperature (Tg), it is usually necessary to employ a higher concentration of reinforcement fibers. Increasing the concentration of reinforcement fibers utilized in this manner can result in the emergence of other less desirable properties such as unacceptable surface finish, easier delamination, greater brittleness, and poor bonding between the cover and the metal outer surface of the roll core, etc. Roll manufacturers struggle to optimize these conflicting properties to achieve a superior roll cover. Failure and inconsistent performance of the synthetic composite roll covers in the field have been and continue to be a common ungovernable problem. Even with recent advances in resin chemistry, synthetic composite roll covers today are best performing at operating conditions wherein the maximum operating temperature does not considerably exceed 250xc2x0 F., and the maximum nip pressure does not considerably exceed 10,000 p.s.i., and wherein the surface roughness of the cover is considerably below 10 Ra micro-inches.
The method and apparatus of the present invention enables the fabrication of roll covers having a tough fiber mat under layer and a high performance or engineering thermoplastic outer layer as opposed to a cover formed of a thermoset epoxy. The prior art method discussed above, wherein an epoxy impregnated strip is helically wrapped over a horizontally disposed roll core, would be unsuitable for the application of a high performance thermoplastic material. This is due to the fact that a thermoplastic extrudate alone would have insufficient viscosity to form into the shape of a cover when applied to a horizontally disposed roll core. Under the method of the present invention, the roll core is oriented vertically rather than horizontally and a mold tape is utilized which surrounds the roll core outer surface in spaced-apart relationship therewith to form a weir or application zone in which the thermoplastic material may be extruded and allowed to set up and harden. Thermoplastic materials have a number of highly desirable properties making them superior to thermoset materials such as epoxy in the fabrication of covered roll cores. This includes a higher glass transition temperature, a higher tensile strength, a higher impact strength, greater smoothness, more uniform surface finish and more homogenous physical and thermal properties. Thus, a roll core covered with a high performance thermoplastic material will achieve superior performance characteristics than one covered with a thermoset material such as epoxy resin. The method and apparatus of the present invention also enables the fabrication of rolls covered with prior art thermoset materials such as those discussed above.
Accordingly, it is a general object of this invention to provide a method and apparatus for covering a roll core that overcomes the disadvantages of prior art outer layer materials.
It is a further object of this invention to provide a cover for a roll core having an under layer that serves as a transitional layer between the roll core and a high performance thermoplastic outer or cover layer.
It is a further object of this invention to provide a cover for a roll core that more effectively adheres to the outer surface of the metal roll core it covers.
It is a further object of this invention to provide a method and apparatus for covering a roll core that minimizes internal stress build-up and thus minimizes the possibility of cracking, delamination or edge lifting.
It is a further object of this invention to provide a method and apparatus for covering a roll core which minimizes the possibility of premature failure of the covered roll core.
It is a further object of this invention to provide a method and apparatus for covering a roll core that results in a polymeric covering having a high tensile strength.
It is a further object of this invention to provide a method and apparatus for covering a roll core that results in a polymeric covering having a more suitable Young""s modulus.
It is a further object of this invention to provide a method and apparatus for covering a roll core wherein the cover has a high glass transition temperature.
It is a further object of this invention to provide a method and apparatus for covering a roll core wherein the cover has a high durability and long lifespan.
It is a further object of this invention to provide a method and apparatus for covering a roll core that is less expensive than prior art methods and devices.
It is a further object of this invention to provide a method and apparatus for covering a roll core wherein the cover has an outer surface that is smoother than prior art covers formed of thermosetting materials such as epoxy resins.
It is a further object of this invention to provide a method and apparatus for covering a roll core that results in a covered roll having a cover that will not fail under extremely high pressures or high heating conditions.
It is a further object of this invention to provide a method and apparatus for fabricating a mold tape assembly having a radius of curvature that can be easily obtained based upon the outer diameter of the covered roll.
It is a further object of this invention to provide an apparatus for fabricating a mold tape assembly that enables the user to determine the radius of curvature of the resulting mold tape assembly.
It is a further object of this invention to provide a mold tape assembly that winds onto itself to form a substantially straight cylinder during fabrication of a covered roll.
It is a further object of this invention to provide a method and apparatus for fabricating a mold tape assembly wherein the height of the resulting mold tape is easily controlled.
It is a further object of this invention to provide a method and apparatus for fabricating a mold tape assembly that is multi-tiered.
It is a further object of this invention to provide a method and apparatus for covering a roll core wherein the cover has a high impact strength.
It is a further object of this invention to provide a cover for a roll core that performs consistently under extremely high pressures, high heating and high speed conditions.
It is a further object of this invention to provide a method and apparatus for covering a roll core wherein the cover has a high compression strength.
It is a further object of this invention to provide a method and apparatus for covering a roll core wherein the cover has a high thermal resistivity.
It is a further object of this invention to provide a method of fabricating and wrapping a glass or other reinforcement fiber mat under layer that minimizes surface irregularities.
It is a further object of this invention to provide a glass or other fiber mat under layer that results in improved radial, axial and circumferential tensile strength.
It is a further object of this invention to provide a glass or other fiber mat under layer that is tightly wrapped over the outer surface of the metal roll core.
It is a further object of this invention to provide a method and apparatus for covering a roll core wherein voids are minimized during the infusion of epoxy resin.
It is a further object of this invention to provide a method and apparatus for covering a roll core with a high temperature polymeric material with a high fiber concentration adhesive layer.
These and other objects of this invention are achieved by providing a method for producing a covered roll having a glass fiber or other fiber mat under layer infused with epoxy or other thermoset resin and a smooth outer layer formed of a polymeric material, preferably a high performance thermoplastic material. The mat is formed of a plurality of layers, each layer having a leading edge that abuts a following edge when wrapped over the roll core, each subsequent layer being affixed to the previous layer along a seam, the seams being spaced apart angularly from one another to prevent irregularities over the surface of the resulting covered roll core. One or more layers of the dry fiber mat such as glass fiber mat may be formed of a two-ply construction, the first ply having fibers arranged in a random pattern, the second ply having aligned fibers oriented either parallel or perpendicular to the roll core central axis. The invention also includes an improved mold tape assembly for use in applying a polymeric cover material over a roll core and an inventive apparatus for fabricating the mold tape assembly. The mold tape assembly is formed of at least two ribbons that are arranged in a substantially parallel and partially overlapping orientation, the ribbons being spot welded together at a predetermined radius of curvature. The apparatus for fabricating the mold tape assembly further includes an inventive device for aligning the ribbons prior to welding and an inventive curvature shoe for disposing the ribbons at a predetermined radius of curvature during welding.